The present invention relates to novel alpha-hydroxy fatty acid derivatives, and external compositions containing the same.
For the purpose of minimizing water loss from the skin and hair and providing them with the smoothness, gloss or the like, oily bases have widely been used. Since these oily bases are applied to portions such as the skin or hair of the human body, the oily substances and body constituting substances preferably resemble each other in components and properties. The oily substances importantly exhibit neither irritation nor toxicity to the skin in view of the safety.
Therefore, there has hitherto been studied about a compound which has a resemblance to human sebum and trials of using ceramides contained exclusively in the human stratum corneum have recently been made (Japanese Examined Patent Publication (Kokoku) No. Hei 6-57651 and Japanese Examined Patent Publication (Kokoku) No. Hei 6-37429). Since a compound having a melting point in the vicinity of the body temperature or the skin surface temperature, or the melting point lower than that temperature, a trial of having an unsaturated bond in a molecular structure of the compound has been made. However, lipid having an unsaturated bond is generally easy to be oxidized by light and heat, moreover, its oxides sometimes induce strong irritation and toxicity to the skin. Therefore, in case such lipid is used in a skin external composition, it is necessary to protect from oxidization. On the other hand, since ceramide contained exclusively in the human stratum corneum is expensive and generally has a very high melting point and a high tendency to crystallize, its application is considerably limited at present.
It has been reported that wax diesters containing alpha-hydroxy fatty acids esterified with fatty acids and higher alcohols are present in the skin surface of animals such as cow, rabbit, cat and the like (T. Nikkari and E. Haahti, Biochim. Biophys. Acta. 164, 294-305 (1968), N. Nicolaides, H. C. Fu and M. N. A. Ansari. Lipids. 5, 299-307(1970)). These wax diesters contain, as principal components, alpha-hydroxy fatty acids having 14 to 22 carbon atoms, fatty acids having 14 to 28 carbon atoms and higher alcohols having 14 to 28 carbon atoms, but wax diesters composed of short-chain fatty acids having 2 to 6 carbon atoms have never been reported. It has never been reported to use wax diesters in external compositions.
An object of the present invention is to provide a novel oily base, which has excellent stability, low melting point, dose not irritate the skin and is superior feel in use, and an external composition contains the oily base.
The present inventors have studied intensively to attain the object described above and found that the alpha-hydroxy fatty acid derivative represented by the following general formula (1). Though the alpha-hydroxy fatty acid derivative is saturated compound, it has low melting point. And it dose not irritate the skin and is also superior in feel in use.
That is, the present invention is an external composition comprising one or more alpha-hydroxy fatty acid derivatives represented by the general formula (1): 
wherein R1 represents a straight-chain or branched alkyl group having 10 to 24 carbon atoms, R2 represents a straight-chain or branched alkyl group having 1 to 31 carbon atoms, and R3 represents a straight-chain or branched alkyl group having 11 to 31 carbon atoms.
The present invention is also an alpha-hydroxy fatty acid derivatives represented by the general formula (2): 
wherein R4 represents a straight-chain or branched alkyl group having 10 to 20 carbon atoms, R5 represents a straight-chain or iso- or anteiso-branched alkyl group having 1 to 5 carbon atoms, and R6 represents a straight-chain or iso- or anteiso-branched alkyl group having 11 to 31 carbon atoms.